Effects of functional groups and species richness on biomass constituents relevant for combustion: results from a grassland diversity experiment

Grassland biomass has been identified as a potential energy source. The combustion of mature and fibrous biomass, as occurs in extensive grasslands managed with low cutting frequencies, is one possible conversion technique. This study tested the relationship between plant diversity and biomass constituents relevant for combustion, as they determine energy content, energy yield and emission and corrosion risks. The biomass from a biodiversity experiment, with a species richness (SR) gradient of 160 species from Central European mesophilic grasslands divided into four functional groups (grasses, legumes, small and tall herbs), was harvested twice a year (in 2008 and 2009). The higher heating value (HHV) was estimated from carbon, hydrogen and oxygen contents to give insight into the energy potential of the species mixtures. The potential risk of emission and corrosion was assessed by analysing ash content, potassium, calcium, magnesium, nitrogen, sulphur and chloride content. HHV was independent of SR, and the overall mean was 18.13 MJ kg(-1) DM. Biomass and gross energy (GE) yield were positively affected by SR. The presence of legumes in a mixture resulted in increased HHV, biomass yield and GE, irrespective of the level of SR. Annual GE varied between 59 (average of monocultures) and 152 GJ ha(-1) year(-1) (mixture of sixty species). The concentration of ash-forming elements was generally high, suggesting a pre-treatment of the biomass prior to combustion. Emission- and corrosion-related constituents were clearly affected by the different functional groups, and sulphur and nitrogen both declined with SR. The results of this study show that high SR in experimental grassland communities is beneficial for the energy output and that legumes play a key role for the energy potential. However, identifying a functional group as being solely beneficial or disadvantageous for fuel quality was difficult.